I get the hunch that the Ghost/Dragon (which according to Serebii is called Giradina) three places below Parukia is the Rayquaza of this game.
Also, the trio above Diaruga are all Psychic, but according to Serebii they are indeed the trio. Now, why they're all Psychic is probalby the same reason the fire starter has an edge on the water starter; Nintendo's being lazy.

Where are these lemmings going? The
Super Nintendo Super Shire! Hop in line and follow them there!